1. Technical Field of the Invention
This invention generally relates to sensors, more specifically to chemical sensors, still more specifically to chemical sensors for metals, and further more specifically to fluorescent chemical sensors for metals based on dipyrrins.
2. Description of the Background Art
Zinc and copper ions both play important roles in biological systems. However, they are also chemical pollutants of the environment. Accordingly, detection of trace amounts of zinc and copper ions is of great importance in environmental and biological analyses. Biosensors play an increasingly important role in understanding biological processes and in determining the physiological state of cells and tissues.
Zinc
Biosensors are essential for the analysis of free Zn2+ because that metal is unresponsive to spectroscopic interrogation. The zinc ion is the second most abundant heavy metal ion and well known to play an important role in cellular events including structural co-factors, regulator of enzymes, DNA binding, catalytic centers and neuronal signal transmission. Zinc is also a well-known metal pollutant of the environment. Too much zinc may reduce the soil microbial activity causing phytotoxic effects3 and it is a common contaminant in agricultural and food wastes. Therefore, the ability to detect and quantify trace amounts of Zn2+ in biological systems as well as in the environment is of great analytical importance.
One way that trace amounts of Zn2+ can be detected and quantified is using chemical sensors. Chemical sensors or chemosensors present distinct advantages over other methods of detection. For example, chemosensors can be designed to selectively detect and quantify trace amounts of metals, such as Zn2+. Chemical sensors use the well known principle of receptor/substrate affinity. The chemosensor is a receptor, which can recognize, detect and measure a target metal by virtue of the chemosensor's selective affinity to the particular metal.
During the past decade, a great deal of effort has been directed toward the development of selective ligands for Zn2+, such as quinoline based ligands, bis(2-pyridylmethyl)amine, acyclic and cyclic polyamines, and others. Looking for the new selective and simple ligands for Zn2+ is still a design and synthetic challenge. The goal is to design a chemosensor molecule or family of molecules which can selectively detect and measure Zn2+ over other metal cations which exhibit high interference from Zn2+.
The prior art discloses several chemosensors for zinc and copper ions. Since Fischer found that dipyrrins can form stable complexes with divalent metal ions in 1924, various complexes of dipyrrins with metals have been prepared.
The internal charge transfer (ICT) mechanism has been successfully utilized in connection with chemosensors. The ratiometric technique has seen a widespread use in biological, polymetric, and sensory materials chemistry. The significant advantages of this technique include reduced artifacts, increased precision and dynamic range with a built-in correction for environmental effects. Consequently, the development of ratiometric Zn+ chemosensors has emerged as an important area in molecular design and synthesis.
A milestone in the development of chemosensors for Zn2+ was 6-methoxy-8-p-toluenesulfonamidoquinoline (TQS), which was first applied to detect Zn2+ in 1987. However, the usefulness of TQS as a chemosensor was hindered by the fact that the excitation wavelength of TQS and its derivatives was not in an optimal UV range. Long-wavelength absorption and emission may be more useful for significantly reducing background absorption, fluorescence and light scattering.
Copper
Like Zinc, Copper has been shown to be a metal of biochemical significance e.g. copper-zinc superoxide dismutase's role in the enzymatic defense against oxygen toxicity. Furthermore, and also like Zinc, copper has been identified as a significant enviromental pollutant. Logically, then, the ability to detect and measure trace amounts of Cu2+ has obvious implication in environmental and biological analysis.
The 8-hydroxyquinoline (8-HQ) moiety has been one of the most useful ionophores, used in a variety of investigations involving metal ion recognition. In recent years, many chemosensors based on 8-HQ have been developed for Zn2+, Cd2+, Pb2+, Hg2+. However, use of 8-HQ derivatized chemosensors for the detection of Cu2+ has not been exploited. Chemosensors that have displayed selectivity for Hg2+ have also shown a much more limited fluorescent change in the presence of Cu2+. Hg2+ has a larger atomic radius than Cu2+, thus by increasing the steric hindrance around the binding site (8-HQ), a significant enhancement in the selectivity for Cu2+ compared to Hg2+ is achievable.
Dipyrrometheneboron difluoride (difuoroboradiaza-s-indacene, BODIPY) has been widely used as fluorescent dyes. They have relatively high fluorescence quantum yields, good photo-stabilities and absorption coefficients. Long-wavelength absorption and emission have been shown to significantly reduce background absorption, background fluorescence, and light scattering. Maximal fluorescence emission was tunable to longer wavelength by extending the conjugation and rigidification of the BODIPY core.